This invention relates to adhesive transfer devices and methods, and, more particularly, to a method and apparatus for applying hot melt adhesive to the parting surfaces or joints of patterns formed of foam material used in metal casting processes.
Thousands of tons of metal castings have been produced by a cavityless casting method employing a foam pattern which is not removed from the mold prior to pouring of the molten metal and evaporates before the metal cools. One process of this type is known as the full mold or lost foam process in which a mold pattern is made from a foam material such as expanded polystyrene. A ceramic wash is applied to the surface of the foam polystyrene pattern, and then the pattern is embedded in dry, unbonded sand which is densely compacted around the foam pattern. A runner system, made from the same polystyrene material, is
Molten metal is poured into the downsprue and flows into the foam pattern. The downsprue, runner system and foam pattern all vaporize but are immediately replaced by the molten metal so that the mold is always full. The sand is prevented from collapsing in on the space which was occupied by the foam pattern by a combination of gas pressure built up as the foam material vaporizes, the refractory coating or ceramic wash applied to the pattern and the rapid replacement of the pattern by the molten metal. The metal solidifies forming a replica of the foam pattern shape.
Aluminum, yellow metal, iron and some steel castings can be produced by the lost foam casting method described above with little change to their chemical content. However, the polystyrene foam contains a high percentage of carbon which can be absorbed by the casting. As a result, castings using low carbon content steel cannot be acceptably produced by the lost foam process. These types of castings are often produced by a ceramic shell process in which a foam pattern is coated with a thin layer of ceramic material after which the foam material is vaporized leaving a ceramic shell. The ceramic shell is then embedded in dry sand in a casting operation similar to that described above.
The foam patterns employed in the lot foam and ceramic shell processes described above are formed by blowing polystyrene beads into a tool through which steam is passed to cause the beads to expand so that they flatten against the tool surface and adhere to one another. After cooling, each solid section of the foam pattern is ejected from the tool for assembly. Particularly in patterns of complex shape, several individual sections of the foam pattern are formed in tools and then joined together along their joints or parting surfaces by adhesives to form the finished pattern.
Attachment of mating parts or sections of the foam pattern is an important aspect of the lost foam casting and ceramic shell processes. The parting surfaces of the foam pattern must be accurately assembled to obtain a casting with an acceptable surface finish and good dimensional accuracy. Voids between the parting surfaces must be eliminated to prevent sand, and the ceramic or refractory material which coats the foam pattern in the ceramic shell process, from entering the space intended for the molten metal. If sand or refractory material is permitted to enter the voids, the surface finish on the casting may be poor. In addition, the adhesive employed to connect the mold parts must be placed on the parting surfaces so that no seepage occurs on the outside of the foam pattern which would create roughness in the surface finish of the molded part.
It has been one practice in the prior art to join parting surfaces of foam patterns by applying hot melt adhesive directly to such surfaces either by hand-held adhesive guns or by adhesive guns manipulated with a robot arm. Manual application of the hot melt adhesive is relatively inefficient and highly labor-intensive adding substantially to the overall cost of producing the foam patterns. While the application of hot melt adhesive by a robot manipulated gun increases efficiency, the cost of such equipment is often prohibitive except in applications where a large number of patterns are to be manufactured.
Another disadvantage with applying hot melt adhesive directly to the parting surfaces of foam patterns, either manually or with a robot arm, is that it is exceedingly difficult to accurately apply the adhesive to intricate patterns. If the adhesive is applied too close to the edge of the parting surfaces of mating sections of the pattern, the adhesive can be squeezed from between the parting surfaces when the pattern sections are assembled and seep onto the exterior of the foam pattern. This produces a rough surface finish on the cast metal part.
One attempt to improve upon the prior methods of applying hot melt adhesive directly to the parting surfaces of a section of a foam pattern is found in U.K. Pat. No. 2,154,906. In this patent, a glue plate is submerged in a hot melt adhesive bath and then raised upwardly into contact with the parting surface of one section of a foam pattern. The top surface of the glue plate is formed with a raised pattern corresponding to the desired adhesive pattern to be applied to the parting surface of the foam section. The foam section is then transferred into engagement with a second foam section, having a mating parting surface, for assembly.
This method of application of adhesive onto the parting surfaces of the sections of a foam pattern eliminates some of the problems in the prior art described above. It is easier to apply adhesive to intricate patterns with this method, and a more even coating is obtained on the parting surfaces of the sections of the foam pattern. However, problems are also created. The apparatus disclosed in the U.K. patent is relatively complex. A mechanism must be provided which extends into the adhesive bath to raise and lower the glue plate without causing leakage. In order to alter adhesive patterns for different sections of the foam pattern, the entire glue plate must be replaced with another glue plate having the desired surface configuration.
Additionally, some problems have been experienced in the prior art with compressing hot melt adhesives after application to the parting surfaces of foam patterns to achieve a uniform layer of adhesive on such parting surfaces. The relatively high viscosity, high surface tension, and quick setting time of hot melt adhesives all combine to prevent the adhesive from readily spreading when the adhesive is applied as a liquid to the substrate. Instead of spreading, the liquid can set up as a thick bead on the structure. Even if the parting surfaces of connecting sections of the foam pattern are quickly compressed after the hot melt adhesive is applied, the adhesive is sometimes difficult to spread resulting in an uneven layer which can reduce the strength of the joint. In addition, conventional hot melt adhesives have a relatively high specific heat in the molten state, which can cause wavering or buckling of the joining surfaces of the foam pattern when applied thereto. If the joints in the foam patterns are distorted, the dimensions of the casting produced may not fall within acceptable tolerances.